CENTRAL NERVOUS SYSTEM. 



71T 



nearly a constant one, and that the results of different weighings are therefore 

 comparable among themselves. 



Interpretations of Weight. Assuming as the simplest case that the num- 

 ber of the nerve-elements composing a given portion of the central system is 

 constant, then differences in the weight of these portions in different individ- 

 uals imply variations in the size of the component cells. The significance of 

 variations in the size of the nerve-elements must be, primarily, that the larger 

 the cells, and especially the larger the cell-bodies, the greater the mass of cell- 

 substance ready at any moment to undergo chemical change leading to the 

 release of energy. On the other hand, if the number of elements is variable, 

 an increase in the number must, in view of the law of isolated conduction, 

 also provide a larger number of conducting pathways. Whether this increase 

 in the number of pathways shall further add to the complication of the sys- 

 tem depends on the localities at which it occurs. Bearing these facts in mind, 

 we may turn to the records of the weight of the encephalon. 



Weight of the Encephalon. The encephalon is that portion of the cen- 

 tral nervous system contained within the skull. The accompanying diagram. 



FIG. 203. Showing the principal divisions of the encephalon made for the study of its weight : 1,. 

 hemisphere seen from the side, fissuration according to Eberstaller ; 2, mid-brain, region of the quad- 

 rigemina ; 3, pons ; 4, cerebellum, or hind-brain ; 5, bulb, or after-brain. Divisions 2, 3, and 5, taken 

 together, form what is designated the "stem" in the tables of Boyd (modified from Quain's Anatomy). 



(Fig. 203) shows the encephalon, together with one manner of subdividing it. 

 Its weight has usually been taken while it was still covered by the pia, but 

 after allowing the fluids to drain away for five minutes or more. As has 

 been stated, sometimes drainage has been facilitated by cutting into the brain ;. 

 hence, when the brain-weight records by any observer are to be discussed, the 

 first question concerns the method according to which the brains were exam- 

 ined, for the weights may be either with or without the pia and with or with- 

 out drainage. 



